Copper ions in aqueous media are toxic to fungi and bacteria because of their ability to destroy proteins in plant tissues. Copper compounds such as basic copper sulfate, copper oxychloride, and cuprous oxide are effective in preventing plant diseases such as late blight of potatoes and tomatoes, leaf blight of celery, and downy mildew of cucumbers and melons. Copper is also a primary component in wood preservation formulations. In the absence of complexing ligands, e.g., amines, nitriles, ammonia, or the like, the solubility of copper in water is extremely low. Therefore, to apply a sufficient amount of copper to a plant, seed, or substrate (such as wood, plastic, and the like), it is advantageous to apply the copper component as micron-sized to sub-micron-sized particles of a sparingly soluble copper salt.
As used here, for convenience and unless otherwise specified, the term “sparingly soluble copper salt” also specifically includes copper hydroxide, though a hydroxide would not normally be classified as a salt. Similarly, while the invention is discussed in terms of a sparingly soluble copper salt, the invention is equally useful for slurries comprising a sparingly soluble zinc salt, and also for slurries comprising copper(I) oxide, zinc oxide, or both. Particulate copper fungicide can be applied more homogeneously and effectively if it can be dispersed in a broad range of water qualities and suspended in the aqueous media for a prolonged period of time. Traditional copper fungicide formulations require large quantities of anionic dispersants or surfactants for dispersion and suspension in a broad range of water qualities. The behavior of particles of copper salts and copper hydroxide in an aqueous slurry is such that a strongly anionic dispersant is required to disperse and stabilize a slurry. Examples of the anionic surfactants or dispersant systems are sodium poly(meth)acrylate, sodium lignosulphonate, naphthalene sulphonate, etc. The term poly(meth)acrylate encompasses polymers comprising a major quantity (e.g., at least 30% by weight, typically at least 50% by weight) of acrylate monomers, e.g., polyacrylates, polymers comprising a major quantity of methacrylate monomers, e.g., polymethacrylates, and polymers comprising a major quantity of combined acrylate-containing and methacrylate-containing monomers.
Another useful class of biocides are cationic organic biocides such as quaternary amines. Synergy between combinations of copper salts and organic biocides are known. A problem exists, however, because a slurry of copper salt particles (including for purposes here, a slurry of copper hydroxide particles) requires a strongly anionic dispersant to achieve a stable slurry. Addition of a strongly cationic organic biocide will destabilize the slurry and cause agglomeration and settling of particles. Quaternary ammonium compounds react with the anionic systems causing coagulation and/or rapid sedimentation of the copper compound. Thus, a copper fungicide formulation comprising quaternary ammonium salts cannot be applied with homogeneity. Formulations to overcome this tendency often utilize extremely high concentrations of anionic dispersants, e.g., the greater of between 5 to 15 grams of surfactants per gram of quaternary ammonium compound, or between 0.8 to 2 grams dispersants per gram of copper-containing particles.
It is desirable to provide formulations comprising lesser amounts of surfactants while maintaining stability when a strongly cationic organic biocide is added to a slurry of copper-containing salts.